How many are there supposed to be all together?

I'm pretty sure it is never stated how many Elder Scrolls there are, so unfortunately, we do not know.

Very, very, many. Have a look around the library in the Ultimate Heist in Oblivion and you can see that the walls are lined with them. And they are supposed to be records of every event of significance in Tamriel in the past and the future, so that would need a lot of scrolls.